In operating an internal combustion engine, combustion data relating to normal combustion characteristics such as the timing for start of combustion (“SOC”) and heat release rate, can be used to improve the combustion quality, engine efficiency and reduce engine out emissions of pollutants. Accordingly, more accurate combustion data relating to at least one of these characteristics is desirable to improve engine operation. Instrumentation for measuring combustion data, such as pressure transducers that employ sensors disposed in the combustion chamber, or emissions sensors that can be located in the exhaust port, or temperature sensors located in the combustion chamber or in the exhaust port can collect data that can be used to indicate combustion characteristics, but it can be impractical to employ such sensors in a mass produced engine, because in addition to being intrusive and/or being subjected to a harsh environment in the location where they are installed, such sensors can also be expensive and lack the durability needed for reliable and prolonged everyday use.
Accelerometer sensors are sold commercially for use as knock sensors for detecting engine knock. Engine knock is considered an abnormal combustion characteristic because it does not occur when the engine is operating normally. Engine knock correlates to violent combustion events that can be caused by premature and uncontrolled detonation of the charge inside the combustion chamber, caused, for example, if an Otto-Cycle engine has been fuelled with a fuel with too low an octane rating, or if spark timing is too advanced, or if deposits in a combustion chamber create hot spots that cause early ignition. In a Diesel-Cycle engine, engine knock can be caused, for example, if fuel injection timing is too early. Engine knock can cause a decrease in engine performance and in severe cases, if not corrected, engine knock can cause serious damage to the engine, including destruction of the pistons, connecting rods, exhaust valves, head gasket and spark plugs or glow plugs. Accelerometers that are employed as “knock sensors” are typically located on an engine's cylinder block and sometimes on the cylinder head. As an abnormal combustion characteristic, engine knock is not a combustion characteristic that is measured in each combustion cycle. Most knock sensors send a base or “no knocking” reference signal to the electronic engine controller and an easily detectable higher signal when engine knock is detected. Because there is a relatively large difference between the reference signal and the signal when engine knock is detected the accuracy of the knock sensor is relatively unimportant. In this respect, knock sensors are only required to make crude measurements since they are configured to detect only engine knock.
An accelerometer sensor that produces a signal that can be used to determine a normal combustion characteristic in a combustion cycle is mounted to a component of the engine, and therefore contains more information than just the information that pertains to a specific combustion characteristic. That is, an accelerometer sensor mounted to an engine can detect deflections caused by many sources other than the monitored combustion characteristic and without processing the raw signal from an accelerometer sensor to filter out signal noise, the raw signal can not be used as a sufficiently accurate indicator of a combustion characteristic. Information indicating a combustion characteristic needs to be accurate for it to be used to better control or diagnose combustion in an internal combustion engine to improve engine performance and efficiency, and/or engine-out emissions. The operation of most types of internal combustion engines can be improved if an engine controller is provided with accurate information about at least one combustion characteristic, such as, for example, information about combustion phasing, which includes the timing for the start of combustion (“SOC”), the combustion rate, which includes the heat release rate as one indication of the combustion rate, the peak heat release rate, the in-cylinder pressure, and engine misfiring. With an accurate determination of a combustion characteristic, engine performance can be improved by adjusting parameters such as, for example, the timing for fuel injection, the fuel injection rate, and the quantity of fuel injected. Furthermore, while this type of information regarding a determined combustion characteristic can be useful for improving the operation of most internal combustion engines, it is particularly useful for a type of engine known as a homogeneous-charge compression-ignition (“HCCI”) engine. Although HCCI engines have not yet been widely commercialized, in recent years significant work has been directed to developing such engines because they offer the potential for higher efficiency and lower engine-out emissions compared to conventional compression ignition Diesel-Cycle engines and spark-ignited Otto-Cycle gasoline engines. For example, at highway cruising conditions the Diesel-Cycle engines in Class 8 heavy-duty trucks operate at about 40% brake thermal efficiency, and these heavy trucks typically achieve only about 6 miles per gallon. HCCI engines could improve engine efficiency significantly, giving a gain in fuel economy. For vehicles that use gasoline-based engines, which are even less efficient than their diesel counterparts, the potential fuel savings is greater still. In addition, because of the potentially lower engine-out emissions from HCCI combustion, HCCI engines might allow future diesel-fuelled engines to avoid selective catalytic reduction and its complicated and expensive system of multiple catalysts.
One of the main challenges delaying the commercial introduction of HCCI engines is the difficulty in controlling HCCI combustion and a method of accurately determining a combustion characteristic such as the timing for SOC or the heat release rate can be used to solve this challenge. For example, a production-ready SOC sensing system could help to enable HCCI combustion over a wide range of conditions. To be successful, such a sensing system should meet the important practical needs of commercial automotive products, namely low cost, reliability and durability.
An accelerometer signal can be conditioned and digitized, for example as set out in co-owned U.S. Pat. Nos. 7,133,761 and 7,200,487, respectively entitled “Method and Apparatus for Controlling an Internal Combustion Engine Using Accelerometers”, and, “System and Method for Processing an Accelerometer Signal to Assist in Combustion Quality Control in an Internal Combustion Engine”.
Some of the challenges associated with methods for determining a combustion characteristic for an engine include engine-to-engine variability, cylinder-to-cylinder variability, and sensor-to-sensor variability. Accordingly, there is a need for a method of determining an engine combustion characteristic that is effective, accurate, and easy to calibrate, even if there can be variability from engine-to-engine, cylinder-to-cylinder and from sensor-to-sensor.